CH20_Lecture18

Chapter 20: Electric Circuits

20.1 Electromotive Force and Current

• Definition of Electric Circuits: An electric circuit consists of an energy source (like a battery) and an energy-consuming device connected by conducting wires that allow electric charges to move.

Battery Functionality

• Chemical Reaction: Within a battery, a chemical reaction facilitates the transfer of electrons from one terminal to another.

• Electromotive Force (emf): The maximum potential difference (voltage) across the battery terminals is known as the electromotive force (emf).

Electric Current Basics

• Definition of Electric Current: Electric current is defined as the amount of charge that passes through a given surface perpendicular to the charge's motion over time.

  • Unit of Current: One coulomb per second equals one ampere (A).

Types of Current

• Direct Current (dc): If charges flow consistently in one direction, the current is termed direct current.

• Alternating Current (ac): When charges alternate direction, it is called alternating current.

Example Calculation: Pocket Calculator

• Current in Calculator: A pocket calculator’s 3.0 V battery outputs a current of 0.17 mA. Important calculations include:

  • (a) Total charge that flows in one hour of use.

  • (b) Amount of energy delivered to the calculator circuit by the battery.

Conventional Current

• Concept of Conventional Current: Conventional current refers to a hypothetical flow of positive charges that would produce the same effect in the circuit as the actual movement of negative charges.

20.2 Ohm’s Law

• Definition of Resistance (R): The resistance of a material is defined as the ratio of the voltage (V) across it to the current (I) flowing through it.

Ohm’s Law

• Ohm’s Law Statement: The ratio V/I is a constant for a given material, where V is voltage and I is current. This relationship defines the electrical resistance.

• SI Unit of Resistance: This measure of resistance is expressed in ohms (Ω).

Resistors in Circuits

• Definition of a Resistor: Any wire or electrical device exhibiting resistance to electrical flow is classified as a resistor.

Example Calculation 1: Flashlight

• Description: The filament of a light bulb functions as a resistor due to its thin wire structure that heats up enough to emit light.

• Flashlight Current: It operates with two 1.5-V batteries producing a current of 0.40 A.

• Resistance Calculation: Calculate the resistance of the glowing filament.

Example Calculation 2: Bagel Toaster

• Resistance of Toaster: A bagel toaster has a resistance of 14 Ω.

• Operation Details: It operates for one minute at a 120-V outlet. Calculate the total energy supplied to the toaster during this time.